Summary This study examined the direct and indirect costs due to acute form of lymphatic filariasis caused by Wuchereria bancrofti to the households in rural communities in Tamil Nadu state in south India. For nearly one-third of the acute adenolymphangitis (ADL) episodes the affected did not seek treatment and for 27% of the episodes they consulted health personnel, underwent treatment and paid for it. On average, the ADL patients spent Rupees (Rs.) 2.35 (US $ 0.07) per episode on treatment, but expenditure was as high as Rs. 32.11 (US $ 0.92) among those who paid. Doctor's fees and medicines constituted 83% of the total treatment costs. Patients with multiple and longer duration episodes and with better living conditions spent relatively more on treatment. The proportion of patients who spent money on treatment was smaller in poorer households, but their treatment costs formed a relatively higher proportion of their income than those of middle and high-income households. The ADL episodes curtailed economic and domestic activities. In 87% of the episodes, the affected were not able to attend any economic activity compared to 37% of the episodes in the case of controls. Patients spent only 0.68 ± 1.91 hours on economic activity compared to 4.40 ± 3.74 hours by the control individuals during the ADL episodes. The sign rank test showed that the mean difference of 3.73 ± 3.81 and 2.14 ± 1.83 hours in the time spent on economic and domestic activity respectively between cases and controls was highly significant (P 0.01). Regression analysis demonstrated that the difference in the time spent on activities is only due to ADL and no socio-economic variable had any effect on it. The cost of treatment and loss in economic activities combined with high incidence in the study communities indicate the extent of the economic burden imposed by the hitherto neglected acute form of lymphatic filariasis and the necessity to control it.